A Ukrainian has replaced a Russian at the helm of the insurgency in eastern Ukraine and declared that he wants "only moral support" from Moscow, as the Kremlin apparently tries to rebut Western claims that it is calling the shots among the rebels.

Many in the rebel ranks decry what they call Russia's betrayal of their cause, but most vow to keep on fighting even as Ukrainian government troops close in on the main rebel stronghold, the eastern city of Donetsk.

In an ominous sign that the fighting may escalate further, the new leader of the insurgency has boasted of hundreds of new recruits and says a lot of rocket launchers and tanks have been seized from a Ukrainian unit.

Ties to Russia

Alexander Zakharchenko, a native of mostly Russian-speaking eastern Ukraine, took over late Thursday as prime minister of the self-proclaimed Donetsk People's Republic, which has declared independence from the central government in Kiev.

He succeeded Alexander Borodai, a Moscow political consultant who reportedly played a role in Russia's annexation of Crimea in March before moving into eastern Ukraine.

Ukraine's eastern regions have strong ties to Russia, and much of its population was alarmed when a new pro-Western government came to power in Kiev with support from Ukrainian nationalists. The change of government was the result of months of street protests that ousted the pro-Moscow president in February.

In another sign that the rebellion may be losing steam, several other rebel leaders with links to Russia have quietly left the region in the past few weeks.

The Russian commanders "are fleeing like rats," said Andrei, a 27-year-old rebel in Donetsk. Like other locals who have joined the separatist cause, he gave only his first name out of fear of retribution either from the rebel leadership or from the Ukrainian authorities.

The new leader of the insurgency vowed to continue the fight but refrained from urging Moscow to send troops, a call issued by many rebel leaders in the past.

Ukraine and the West have accused Moscow of beefing up its military on the border, dispatching what NATO estimates is 20,000 troops to the border of Ukraine.

The Russian Defense Ministry announced late Friday that it has wrapped up military exercises in southern Russia that the U.S. had decried as a provocative step.

Speaking Friday at a U.N. Security Council meeting, U.S. Ambassador Samantha Power warned Russia that any further intervention in Ukraine, including under the pretense of delivering humanitarian aid, "would be completely unacceptable and deeply alarming, and it would be viewed as an invasion of Ukraine."

Donetsk surrounded

The Russian government has denied Western accusations of backing the Ukrainian mutiny with weapons and soldiers.

Ukrainian troops routed the insurgents from smaller towns this month and have now encircled Donetsk. An estimated 300,000 of the city's 1 million residents have fled.

The Donetsk city council said four apartment buildings in the city were damaged by artillery barrages overnight, killing at least three civilians and wounding 10 others. Shocked residents gathered at the site in the morning, with some leaving flowers on the pavement to commemorate the victims.

Zakharchenko said the shelling has swelled the rebel ranks, with more than 700 men showing up to volunteer in recent days.